We have developed a stopped-flow flow cytometer with subsecond resolution to be used in kinetic analysis of cell activation. This instrument is extending the application of flow cytometry to important biological molecules that interact with cell surface receptors or can be linked to beads. The device is making it possible to examine the kinetics of interaction of a variety of macromolecular assemblies. In year 17, we took delivery of a specially designed Bio-Logic five syringe mixing device, tested it on the flow cytometer, and began to incorporate DiDAC into the instrument. We suggested modifications to the mixing system to allow sample and sheath delivery over an extended range of flow rates. The modified mixing system is expected to be delivered in May 1999. With funds provided in year 18, the new mixing system will be integrated into a Cytomation flow cytometer. The entire system is expected to be operational during year 18, in keeping with the time line proposed in the NFCR renewal application for years 16-20. A number of collaborative projects described below have advanced to the stage where they can be examined with the new instrument.